Better handling of optional fields

[mohansammeta]

Consider the below example :

{
  "my_structs": [
    {
      "struct_with_a_and_b": {
        "a": 123
      }
    },
    {
      "struct_with_a_and_b": {
        "a": 111,
        "b": 123
      }
    }
  ]
}

Generated constant will have "b" value generated but it is not declared as field in "struct_with_a_and_b".